CHAP. I. ELEMENTARY ORGANS. 251 



perly so called, are concerned. Whether or not ducts have a 

 different function is uncertain ; it is probable, however, from 

 the extreme thinness of their sides, that they are reallj^ filled 

 with fluid when full grown, whatever may have been the case 

 when they were first generated. 



In regard to the functions of aii-cells and lacunae, it may 

 be sufficient to remark, that in all cases in which they form a 

 part of the vital system, as in water plants, they are cavities 

 regularly built up of cellular tissue, and uniform in figure in 

 the same species ; while, on the other hand, where they are 

 not essential to vitality, as in the pith of the walnut, the rice- 

 paper plant, the stems of Umbelliferas, and the like, they are 

 ragged, irregular distensions of the tissue. 



In the former case they are intended to enable plants to 

 float in water ; in the latter, they are caused by the growth of 

 one part more rapidly than another. 



